A continuing problem in the production of refractories designed for emplacement by ramming, gunning, shovelling or casting is that they must be consolidated into monoliths by firing to produce sintering or partial melting of the mass. In order that this melting will not continue during subsequent service, refractories must be designed so that sintering will stop after a given time at temperature, or refractories must be used whose sintering temperatures exceed the furnace operating temperature.
A commonly used method of providing controlled sintering at operating temperatures is to take advantage of the fact that finely communited refractory grains tend to sinter below their fusion temperatures. The disadvantages are that the sintered zone usually is quite thin and the time for sintering is usually burdensomely prolonged.
Recourse to using grains whose sintering temperature is above the normal operating temperature of the furnaces they are used in results in some other disadvantages. Among these are the following: (1) most metallurgical furnaces are commonly operated at temperatures which are limited not by the monolithic refractories usually used in their hearths, but by the maximum service temperature of exposed brickwork in the furnace walls and roofs, therefore, little or no sintering of the refractory grains in the hearths would take place. Even among the rare cases where such sintering can be made to occur, the sintered face would be relatively thin and normal erosion would sooner or later expose unconsolidated grains which would simply fall (or float) out of place, (2) electric arc, electric induction and basic oxygen furnaces derive their heat by generating it within the metallic charge. Heating these furnaces to temperatures above their normal operating temperatures before putting them in service is, at best, time consuming, awkward, and expensive, and sometimes virtually impossible.
To a degree, chemically bonded (unfired) brick is subject to the same disadvantages.
Recently, some thermo-chemical approaches have provided individual solutions. These involve preparation of refractory mixes or bodies, which in the unfired state or in normal atmospheres remain granular or plastic -- or simply `glued` together by adhesives or cements. These materials, upon firing to temperatures at or below normal operating temperatures or exposed to other than normal atmospheres, react to form chemical species which are stable at temperatures well above the temperatures at which they will be required to serve. To be successful the new, thermally-stable chemical species must hold the mass together. This can be accomplished by polymerization, by direct chemical combination of two or more different substances which were formerly physically separate, or by development of a continuous, stable and strong matrix which can adhere to and thus hold together large grains (aggregate) of inactive refractory materials.